This invention relates to numerically controlled machine tools such as milling machines providing relative positioning and movement between a workpiece and tool based on revolute motion.
Current numerically controlled milling machines rely on linear sliding mechanisms to position the workpiece relative to the spindle, and provides motion for cutting. Such linear motion requires very high precision linear bearing elements which are expensive to make and to maintain. Long and true flat surfaces are also expensive to produce and maintain (often they are ground by hand), and the bearings used are made to very high tolerances and in limited production.
A milling machine based on the use of revolute joints is believed to be capable of overcoming many of the disadvantages inherent in the use of sliding motion. Rotational joints are generally less expensive, easier to produce to high tolerances, and require less maintenance. Like a robot arm, a machine tool with stiff arms moveable about rotational (or revolute) joints should be capable of producing any motion with quite high accuracy and precision. As a conventional mill can cut complex curved surfaces using numeric control of several linear axes, a revolute mill could use numeric control to produce both straight and curves surfaces with rotational motion. The design concept can easily be implemented in machines having from two to five axis of revolute motion.
One advantage of the use of revolute motion in the control of tool position is the potential for lower cost. The production of tapered roller bearings is of much higher volume than linear bearings, and their cost is lower. At the same time they can easily be pre-loaded to provide very high stiffness and provide very accurate rotation travel. Similarly, the production of circular shafts and bores for mounting the bearings is typically easier and less expensive than the production of flat surfaces. Very accurate boring and turning machine tools currently exist, but linear systems must often be hand scraped and lapped if high accuracy is required. Finally, because less constraints exist on the size of the machine and its load bearing members, the possibility exists to make the machine stiffer than conventional milling machines.
Besides advantages in the construction of a machine tool, the use of revolute joints also has advantages in view of wear and maintenance. The use of revolute motion results in less degradation of accuracy with bearing wear and uses more commonly available, less expensive replacement bearings. When being reconditioned, the revolute motion machine requires less machining operations to true the precision surfaces than a linear travel machine would. A milling machine typically operates near the center of its travel at all times. The result of this is the center portion of the bearing surfaces or the bearings at the center of the table tend to wear out more quickly than those at the edges. This wear results in inaccurate table motion and thus machine reconditioning or adjustment becomes necessary. To address the condition, the surfaces that the bearings run on must be trued, the bearings must be replaced, and the machine must be adjusted. This can be very expensive.
Using revolute joints with highly pre-loaded bearings, the wear of the machine has less effect on accuracy. As the machine wears the pre-load decreases, but until a severe amount of wear has occurred the machine will still run as true as it did initially. This means that bearing replacement occurs less often than with linear systems, even if the linear systems are also pre-loaded (a difficult and expensive proposition). When the bearings must be replaced, they generally can be bought locally. No machining is necessary. The system must be adjusted after bearing replacement, as must a conventional milling machine.
Another advantage of the use of revolute joints is the small package size. A typical milling machine moves the table, and hence requires clearance on each side of the table. In the case of a revolute mill where the table only moves perpendicular to the work surface, the spindle is contained within the boundaries of the table, and thus less clearance is needed around the machine. The result is slightly lower work space footprint requirement than a conventional milling machine for equal workpiece sizes.
A final advantage of the revolute concept is its flexibility in application resulting from several factors.
First, the cutting head work holding elements of a revolute machine can easily be made to swing outside the normal working position, so that the machine can serve as its own loader and tool changer. Second, because the drive and bearing elements of revolute machine are located away from the work area, space around and under the work area can be used for additional axes of articulation. Thus, it will be relatively easy to build machines serving universal purpose, such as combined milling and turning machines.
Proposals have been made in the past to provide machine tools based on the concept of multiple axes of revolute motion. These efforts have largely been equivalent to a robot arm which models the human arm in which long arms are provided between revolute motion joints. Torque is applied by tangential actuators at the joints. These devices suffer the disadvantage of having a relatively low stiffness. In milling machine tool applications stiffness is of paramount importance. Although active tool and workpiece positioning systems based on closed loop control could be provided as a means of overcoming a lack of structural stiffness, such systems cannot completely overcome such an inherent defect. Such systems would possess low resonance frequency conditions which could not be fully addressed through feedback control systems. In order to be commercially acceptable a machine tool such as a milling machine based on revolute motion must possess exceptionally high rigidity in order to at least equal currently available numerically controlled milling machines based on translational movement.
In accordance with the revolute motion machines of this invention, high structural stiffness of the machine tool is provided through incorporation of the number of features. The arms of the machine tool according to this invention utilizes the concept of a structurally efficient three-dimensional truss. When viewing one of the revolute arms in a direction parallel to its axis of rotation, a three sided truss structure is defined with the revolute joint bearing defining one corner of the truss, and the tangential actuator drive point and the driven end defining the other two corners of the triangle truss structure. Depending on the function of the arm, the driven end is coupled to a workpiece supporting table, spindle, or another arm. Preferably the distance between the tangential actuator drive point and the axis of rotation is large as compared with the distance between the axis of rotation and the driven end, as a means of enhancing stiffness. The arms of the machine of this invention also define an efficient three-sided truss when viewed perpendicular to the axis of rotation. A pair of bearings at the revolute motion axis are widely separated thus defining a long length side of the truss structure. These features provide high stiffness in directions tangent to the motion of the driving end and in a direction parallel to the axis of rotation. The high column stiffness of the arms provides radial stiffness.
A high degree of stiffness of the machine tool according to this invention is also enhanced through use of specially designed tangential actuators which provide low backlash and high stiffness. One type of actuator believed suitable for such application is a partial rotary driven cable actuator as described in this specification.
In addition to the design of the revolute motion arms themselves, aspects of this invention also relate to particular manners of coupling of various revolute motion arms to provide the number of degrees of freedom necessary for a given machine tool application.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.